A stream of particle material can be accelerated on an open rotor with the aid of centrifugal force. With this technique the material is fed into the central part of the rotor that rotates rapidly about a vertical axis of rotation, which material is then collected by one or more sliding members, which are positioned on the rotor, hence the name open rotor, which is known from U.S. Pat. No. 6,149,086. The sliding member is provided with a sliding face that stretches into the direction of the outer edge of the rotor between an inner face that is directed towards the axis of rotation and stretches in vertical direction between a bottom edge and an top edge, and an outside face that is directed towards the housing that surrounds the rotor. Particles are collected at the inner face by the first part of the sliding face, then accelerated along the slicing face under influence of centrifugal force and then throw outwards to collide with a stationary impact member that is positioned around the rotor, when the material is crushed. Instead of allowing the material to impinge directly on a stationary impact member, it is also possible to allow the material to impinge first with a co-rotating impact member that is associated with the sliding member, the material being simultaneously loaded and accelerated during the co-rotating impact, with which velocity the material is then thrown from the rotor for a second impact when it strikes the stationary impact member. Such rotor is known from PCT/NL97/00565, which was drawn up in the name of the applicant.
The material is normally fed onto the centre of the open rotor with the aid of an metering member that is provided with a circular metering opening, for example an metering pipe. With an open rotor the outer edge of the metering opening has to be positioned closely to the top edge of the inner face of the sliding members, because the feed material has a strong tendency to escape when there is space between the outer edge of the metering opening and the top edge of the inner face of the rotating sliding members, and this can strongly disturb the accelerating process. The reason for this is that the rotating flow creates a rather concave vortex above the rotor—essentially in a way tea rotates in a cup—and this vortex generates a rather strong upward flow that exerts a pull on the particles in the centre space of the rotor; that is, until they are collected by the sliding members. It can however not be avoided that the metering opening wears off and this requires that the position of the metering member has to be adjusted close to the top edges of the inner faces each time when the sliding members are exchanged because of wear. Such wear can be irregular in which case correct positioning above the sliding members is no longer possible and early exchange of the metering member is required.
Escape of particles because of vortex pulling can be hindered in that the inner face of the sliding member can be angled outwards, that is, with the bottom edge of inner face positioned at a greater radial distance from the axis of rotation than the top edge—such sliding member is known from WO0145846, which was drawn up in the name of the applicant. When the angled inner face rotates it describes essentially a downward widening truncated cone that improves the collection of the material in that it hinders upwards movement of particles. The known sliding member has, however, a problem in that particles and hence wear concentrate at the bottom edge of the inner face and from there along the bottom edge of the sliding face causing irregular wear patterns and non-effective use if wear material while total capacity is reduced. Furthermore, intense wear develops along the rotor face in front of the sliding members where the material is collected.
Another disadvantage with the known sliding members is that the sliding face has to stretch along a rather long distance, that is, from as close as possible to the metering opening towards the outer edge of the rotor. This requires much wear material and limits the number of sliding members that can be positioned around the centre part of the rotor because ample space is required between the inner faces of the sliding members to allow unhindered transport of the material to the respective sliding faces.
Particles that are metered into the centre space of the rotor do not feel the fast rotating face, in a way a magician pulls a table cloth fast from underneath the table wear that remains in place. This means that the particles move outwards in essentially radial direction, that is, seen from a stationary point of view. However, seen from a viewpoint rotating with the sliding member, the particles move outwards along a spiral path as is described in more detail in PCT/NL97/00565, which was drawn up in the name of the applicant. It is this spiral movement that applies, when the particles are collected at the collecting face of the sliding member, to be further accelerated along the sliding face. This means that the particles have to change direction when they are collected by the sliding face. So, the collection process involves essentially an impact of the particle with the sliding member during which impact the particles suddenly change direction and are instantly accelerated to take the velocity of the collecting face. With radial directed sliding members the change of direction is normally between 60° and 80°. The acceleration depends on the speed of the collecting face and increases with radius and rotational velocity.
Another important development with sliding members involve cavities and pockets along the sliding face as well as ceramic inserts to prolong operation time. Such sliding member is known from WO 02/09878.
Vortex pull, impact and change of direction disturb the collection process and cause heavy wear along the rotor face and along the metering opening. It is for this reason that the sliding members have to be positioned as close as possible to the axis of rotation with the top edge of the inner face as close as possible to the outer edge of the metering opening—this limits but does not avoid these problems and requires constant adjustment of the metering opening, sliding members with long sliding faces and hence much wear material and this limits the number of particles that can be placed on the rotor.